Bring a large pot of salted water to a rolling boil, add green beans and cook
until just tender-crisp, 3 to 5 minutes. Drain immediately unto a colander and
pour ice water over beans to stop the cooking action. Drain well.
In a blender or food processor put the garlic, basil, salt and ground pepper.
Pulse on and off, then add the mustard and vinegar. Pulse until smooth. Add the
oil very slowly in a thin stream with the machine running, just until blended.
Place the beans in a serving bowl and pour the vinaigrette over them, Toss to
coat thoroughly. Garnish with the scallions and walnuts.